There are plenty of posts on the site about these. The search engine should be your best friend.
Both Fish and Flax oil are sources of Omega 3's, the main difference being that Fish Oil contains the EPA/DHA components in the Oil, whereas Flax contains Alpha Linolenic (sp?) Acid, which is converted in the body to EPA/DHA so and after conversion a serve of flax contains less EPA/DHA that the same size serve of fish oil.
Both have other benefits as well so I would suggest seeking out some of the articles on this site from the authors that know more about the subject.